The majority of new HIV cases in the US occur among youth under 25 years old. Among infected youth, most are part of marginalized populations. Transgender female youth are a stigmatized and marginalized youth population disproportionately at risk for HIV. The two HIV risk studies of transgender female youth to date have found that one in five are HIV-infected before the age of 25. In San Francisco, transyouth have the highest HIV prevalence of any population, the highest proportion of AIDS cases among youth, and the fastest rate of death due to AIDS in the city. Despite their extraordinary risk for HIV, transgender female youth are understudied in the scientific literature. Gaps in the data exist due to cross-sectional study designs with convenience samples of transyouth who represent only the most risky sub- populations. In part due to these gaps, to date there are no HIV prevention interventions that have been developed specifically for this population. We propose to conduct a longitudinal qualitative and epidemiological study of 300 transgender female youth ages 16-24 years to examine HIV-related risk behaviors and identify protective factors for resilience. We will reach and retain participants by taking an innovative approach to adapting respondent driven sampling using socially interactive technologies. The Specific Aims of this application are to adapt and refine measures of protective factors and to explore resilience among transyouth, determine risk factors specific to development that explain engagement in HIV-related risk behaviors among transyouth, and identify protective factors for HIV-related risk behaviors among transyouth. The methods to be investigated, refined, and applied for this research are critical to the evidence base of HIV prevention for diverse populations of MSM and other marginalized groups. This proposed project will fill important gaps in the literature by providing the first set of causal data from a diverse, representative sample of transyouth addressing both the challenges this population faces and the strengths they possess to ultimately intervene to reduce HIV infection.